(CNN)Two men in a pickup truck abandoned a dog at a Louisiana gas station and drove off, leaving the animal desperately chasing after them, an animal advocacy group says.

"The first man told the dog to: 'Go! Go over there!' The man in the back of the pick-up proceeded to pull up the tail-gate of the truck," Lorie Hollis, a witness, said in a statement Monday from the Humane Society of Louisiana. "Appearing confused, the dog jumped up and put his paws on top of the tail-gate. The truck sped away, leaving the dog behind"

Hollis said she got in her car and went after the truck and the dog, described as a black German Shepherd-Lab mix with a small patch of brown under its neck.

"The truck crossed over two lanes of traffic, and still the dog followed," she said.

Eventually she lost track of the vehicle and its canine pursuer.

'A cowardly act'

"My heart is broken, and I am in shock that someone would treat a dog like this," she told the Humane Society.

But she managed to draw more attention to the case by posting on Facebook a cellphone photo she took of the dog running after the truck. The post was rapidly shared hundreds of times.

Now, the Humane Society is offering a reward of $250 for information leading to the arrest of the owner and the safe return of the dog.

"If people feel they can no longer care for a pet, all they have to do is surrender him or her to a nearby rescue group or shelter, rather than abandon the poor animal at some random location, where their chances of survival are almost zero," said Jeff Dorson, the director of the society. "This is truly a cowardly act and a punishable offense."